The imported cabbageworm adult is one of the most common butterflies in the Northeast. The velvety-green caterpillar of this butterfly feeds on the leaves of cabbage, cauliflower, broccoli, Brussels sprouts, turnips, radishes, kale, lettuce and weeds of the mustard family. The imported cabbageworm is one of the most damaging and destructive enemies of these plants. The caterpillars chew irregular holes in the leaves and usually eat their way into cabbage heads from the bottom. The fecal pellets of the caterpillars will also stain cauliflower. These caterpillars can be controlled with microbial insecticides based on strains of Bacillus thuringiensis effective against caterpillars, or row covers can be used to keep the adults from laying eggs on plants in the garden.

Description:

The first sign of the imported cabbageworm is the 40 mm (1.5 inch) long white butterfly, first seen in early spring. The butterfly has black-tipped front wings with one or two black dots and one black dot on each hind wing. The females fly during daylight hours looking for suitable plants on which to lay their yellowish, elongated eggs, singly, on the underside of leaves. The velvety-green caterpillar has a faint yellow stripe down its back and a row of faint yellow spots on each side. When fully grown, the caterpillar is slightly more than 25 mm (1 inch) long.

The eggs laid by the butterfly on the underside of leaves hatch in five to seven days. There can be as many as three generations of the imported cabbageworm in the Northeast. Adults may be seen almost anytime during the summer.

The pupa, or chrysalis, of the imported cabbageworm is green or tan, 20 mm (3/4 inch) long, and attached to the plant or other surface by silken threads. The imported cabbageworm overwinters as a pupa, attached to any convenient stem, branch, fencepost, or other hard surface in the area.

Damage:

The eggs, larvae, and pupae are found on plants in the cabbage family, including broccoli, cauliflower, collards, kale, Chinese cabbage, and Brussels sprouts, as well as cabbage. The larvae also feed occasionally on turnips, radish, mustard, and even lettuce. When large larvae are feeding, you will find wet masses of dark green excrement dropped on the surfaces of leaves below. It is common for high larval populations in July and August to damage plants so severely that they die or become unmarketable. The larvae eat large holes in the leaves. As they feed, they frequently move toward the center of the plant in order to feed directly on cabbage heads and on leaves shading cauliflowers from the sun. They are also often found inside broccoli heads when they are cooked.

Control Methods:

Natural: Controlling weeds around the garden, especially plants of the mustard family, should help decrease the numbers of this pest. Destroying and removing the remains of plants in the fall, as well as fall tillage, reduces the number of over-wintering pupae. Hand-picking the larvae may be labor intensive but can significantly reduce the numbers and keep damage down. Early Globe, Red Acre and Round Dutch cabbage have shown some resistance to cabbageworm. Several plant breeding programs are working on developing glossy varieties of cauliflower, broccoli and other crops that will be resistant. Barriers such as row covers or a nylon stocking stretched over a cabbage head can keep the butterflies from laying eggs on the plants. The imported cabbageworm has many insect natural enemies, including several parasitic wasps and predators of eggs and young larvae. You may find dead caterpillars with a large mass of white or yellow cocoons nearby. The cocoons are from a parasitic wasp introduced to this country specifically as an enemy of imported cabbageworm.

Insecticides: B.t. (Bacillus thuringiensis), an insecticide derived from a bacterium and sold as Dipel, Bactur, Sok-Bt or Thuricide, is the preferred control for the imported cabbageworm. B.t. is less effective under cooler conditions. Regardless of what is used, the smaller the caterpillars are, the easier they are to kill. Insecticides must be used in late afternoon or early evening to protect bees. This is also the time when the caterpillars feed most actively, making the insecticide most effective. Follow instructions on the label.

Adapted from the University of Maine Cooperative Extension and the Connecticut Agricultural Experiment Station, 1999